Particle detection device, lithographic apparatus and device manufacturing method

ABSTRACT

To enable differentiation between a particle and a ghost particle, a detector system resolves radiation from a ghost particle from radiation from an actual particle. The detector system outputs at least two detector signals corresponding to intensities of radiation being incident on different parts of the detector system or the detector system outputs at least two detector signals corresponding to intensities of radiation with different wavelengths being incident on the detector system. If radiation is received from a ghost particle, not each of the at least two detector signals has a level above a predetermined threshold level, whereas radiation received from a particle results in the signals having substantially a same level above a threshold level. The detector system may include a radiation detector device configured to generate the first detector signal in response to radiation incident on at least one predetermined part of the radiation detector device and a radiation blocking assembly configured to prevent radiation not originating from a particle from being incident on the predetermined part of the detector device.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/957,752, filed Oct. 5, 2004, the entire contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a particle detection device, alithographic apparatus including a particle detection device and adevice manufacturing method.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, such as a mask, may be used togenerate a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g. includingpart of, one, or several dies) on a substrate (e.g. a silicon wafer)that has a layer of radiation-sensitive material (resist). In general, asingle substrate will contain a network of adjacent target portions thatare successively exposed. Conventional lithographic apparatus includeso-called steppers, in which each target portion is irradiated byexposing an entire pattern onto the target portion at once, andso-called scanners, in which each target portion is irradiated byscanning the pattern through the projection beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction.

The imaging of the pattern including small structures, possiblyprotected by a pellicle, is very sensitive to dust and othercontamination of the patterning device and substrate. Therefore, beforeimaging, the patterning device (and/or the pellicle protecting the smallstructures thereof) and substrate are tested for contamination, inparticular for particles. In conventional lithographic apparatus, aparticle detection system directs a beam of radiation, in particular(but not necessarily) monochrome radiation, i.e. radiation havingsubstantially one wavelength, on a surface of an object, for example,but not limited to, the patterning device or the substrate. The objectand/or the beam move in order to scan the surface of the object. Whenthe beam of radiation engages the surface of the object, the radiationis partially reflected according to physical laws of reflection (an exitangle is identical to an angle of incidence with respect to a fictitiousline perpendicular to the surface (the normal)). Another part of theincident radiation may enter the object, such as the patterning deviceor substrate, and is refracted. In both cases, the beam isanisotropically redirected. When the beam of radiation engages acontaminating particle, the radiation is scattered, i.e. reflectedisotropically.

A radiation detector is positioned with respect to the surface and thebeam of radiation such that radiation reflected on the surface is notincident on the detector, but a part of the radiation scattered, i.e.being reflected in substantially every direction, by a particle or othercontamination is incident on the detector. Thus, the detector receivesradiation only when the beam of radiation is scattered by a particle orother contamination.

A part of the radiation incident on the surface of the object enters theobject and is refracted, as above mentioned. Inside the object, the beammay be refracted and/or diffracted by the chrome pattern and/orreflected one or more times. Depending on a number of parameters, suchas the material, the size, the geometry, and the like, a part of theradiation that entered the object will leave the object again in thedirection of the detector. In that case, the detector detects radiationnot being scattered by a particle. As a result a detection circuitreceiving a signal from the detector determines that a particle ispresent, although no particle is actually present. Such a detected, butnot actually present particle will hereinafter be referred to as a ghostparticle.

In other conventional systems for detecting particles, a microscope maybe used. Such systems use a microscope to scan the surface and mayperform a detailed analysis of any detected particle. However, suchsystems are expensive and less suitable for mere in-line detection ofparticles.

SUMMARY OF THE INVENTION

Embodiments of the invention include a simple and cost-effectiveparticle detection system for in-line detection of particles on asurface of an object, which system is capable of discriminating betweena signal from a particle and an erroneous signal, i.e. a ghost particlesignal. Radiation from a ghost particle may be spatially, or otherwise,resolved from radiation from an actual particle.

According to an embodiment of the invention, there is provided aparticle detection system configured to detect a particle on a surfaceof an object, the system including a first radiation source configuredto generate a beam of radiation having a first wavelength, the beambeing directed at a detection area at the surface of the object; aradiation detector system configured to output at least two detectorsignals corresponding to an intensity of radiation received from thedetection area being incident on the detector system; a detectioncircuit configured to determine from the at least two detector signalswhether a particle is present on the surface of the object; wherein thedetection circuit is configured to compare the at least two detectorsignals with a predetermined threshold level, and to indicate that aparticle is present on the surface of the object, when each of the atleast two detector signals has a level above the threshold level.

The particle detection system according to embodiments of the presentinvention is capable of discriminating between isotropic andnon-isotropic radiation. As mentioned above, radiation scattered by aparticle is isotropic radiation. Radiation coming from the radiationsource and being refracted, reflected and diffracted internally of theobject and thereafter being incident on the detector system isanisotropic as will be explained below in relation to the drawings.

Starting with a beam of light directed at a detection area at thesurface of the object, isotropicly redirected radiation may bedistinguished from anisotropicly redirected, i.e. scattered, radiation,since anisotropic radiation has a predetermined cross-section, i.e. alimited cross-section, for example a circle, rectangle or any othershape. Such a limited beam results in a corresponding limited spot, whenincident on a surface, such as the surface of a detector or any othersurface in the trajectory of the beam of radiation. Isotropiclyscattered radiation, on the contrary, does not result in such a limitedspot. Isotropicly scattered radiation is directed in virtually anydirection. Thus, the isotropic part of the radiation incident on thedetector system may be distinguished from an anisotropic part byevaluating whether any concentrated light spot is present in the beam ofradiation. The discrimination based on the distribution and location ofradiation on a surface of the detector system will hereinafter bereferred to as spatial discrimination.

The radiation incident on the detector system may be detected such, thatat least two signals are generated, for example using more than onesignal from one detector or one signal from each of more detectors, fromwhich it may be deduced whether the incident radiation includesisotropicly redirected and/or anisotropicly redirected radiation.Generating at least two signals from the radiation on the detectorsystem is performed such that spatial information may be derived fromthe combination of the at least two signals. A detection circuit maydetermine whether the incident radiation originates from a particle orfrom a ghost particle by e.g. spatially resolving radiation from a ghostparticle and radiation from an actual particle.

A cross-sectional shape of the beam of radiation coming from theradiation source may be round (a circle) or may have any other shape, aline for example. The radiation having a first wavelength may bemonochrome radiation, but may also include radiation having a wavelengthin a certain range. In particular, when only one radiation source isused, even white light, i.e. radiation including many wavelengths in thevisible range, may be used.

The radiation detector system may include a simple radiation sensitivedevice or it may be a one or two-dimensional radiation-sensitive devicesuch as a 1D (linear) or 2D (planar) CCD-element or one ortwo-dimensional radiation-sensitive photo diode (PSD).

The particle detection system may be configured to determine a size of adetected particle based on the detector signals. As mentioned above, thedetector system may comprise a detector device comprising an array ofdetector pixel elements (e.g. a 2D (planar) CCD-element), each detectorsignal of the plurality of detector signals corresponding to anintensity of radiation incident on at least one detector pixel element,thereby providing a particle image of a detected particle, the imagecomprising an array of image pixels, wherein the detection circuit isconfigured to: detect whether a detected particle is small or largebased on the particle image; determine the size of a small particlebased on the radiation intensity incident on at least one detector pixelelement; and determine the size of a large particle based on imagefeatures of the particle image.

According to an embodiment of the invention, there is provided alithographic apparatus including an illumination system configured tocondition a beam of radiation; a support structure configured to supporta patterning device, the patterning device serving to impart the beam ofradiation with a pattern in its cross-section; a substrate tableconfigured to hold a substrate; a particle detection system configuredto verify that substantially no particles are present on a surface ofthe patterning device or the substrate; and a projection systemconfigured to project the patterned beam onto a target portion of thesubstrate, wherein the particle detection system includes a detectioncircuit, which is configured to compare at least two detector signalswith a predetermined threshold level, and to indicate that a particle ispresent on the surface of the object, when each of the at least twodetector signals has a level above the threshold level.

According to a further embodiment of the invention, there is provided adevice manufacturing method including providing a substrate; providing abeam of radiation using an illumination system; using a patterningdevice to impart the projection beam with a pattern in itscross-section; projecting the patterned beam of radiation onto a targetportion of the substrate; and verifying that substantially no particlesare present on a surface of the patterning device or the substrate usinga particle detection system, wherein the particle detection systemincludes a detection circuit, which is configured to compare at leasttwo detector signals with a predetermined threshold level, and toindicate that a particle is present on the surface of the object, wheneach of the at least two detector signals has a level above thethreshold level.

A device manufacturing method including projecting a patterned beam ofradiation onto a target portion of a substrate; and detecting a particleon a surface of an object, the detecting including providing a beam ofradiation directed at a detection area at the surface of the object,detecting the beam of radiation scattered by a particle; outputting aplurality of signals corresponding to an intensity of the detected beamof radiation, and comparing the plurality of signals with apredetermined threshold level to determine whether a particle is presenton the surface of the object.

A particle detection system configured to detect a particle on a surfaceof an object, in accordance with an embodiment of the invention includesa first radiation source configured to generate a beam of radiationhaving a first wavelength, the beam of radiation being directed to thesurface of the object; a radiation detector system configured to outputa plurality of detector signals corresponding to an intensity ofradiation incident on the detector system; and a detection circuitcoupled to the radiation detector system and configured to determinefrom the plurality of detector signals whether a particle is present onthe surface of the object; wherein the detection circuit is configuredto compare the plurality of detector signals with a predeterminedthreshold level, and to indicate that a particle is present on thesurface of the object, when the plurality of detector signals has alevel above the threshold level.

A particle detection system configured to detect a particle on a surfaceof an object, in accordance with an embodiment of the inventioncomprises a radiation source configured to generate a beam of radiation,the beam of radiation being directed to the surface of the object; aradiation detector system configured to output at least a first detectorsignal corresponding to an intensity of radiation incident on thedetector system; and a detection circuit coupled to the radiationdetector system and configured to determine from the at least onedetector signal whether a particle is present on the surface of theobject. The radiation detector system comprises a radiation detectordevice for generating the first detector signal in response to radiationincident on at least one predetermined part of the radiation detectordevice; and a radiation blocking assembly for preventing radiation notoriginating from within a detection range around the surface of theobject from being incident on the predetermined part of the detectordevice. Thus, the radiation from a ghost particle and radiation from anactual particle is spatially resolved in a plane of the detector of thedetector system. As a result, the detector signal is only dependent onradiation originating from scattering by a particle on the surface ofthe object.

In the above embodiment, the radiation detector system may be configuredto further output at least a second detector signal for indicatingwhether radiation is incident on the detector device outside said atleast one predetermined part of the detector device. Thus, a secondsignal may be indicative for the presence of radiation originating froma ghost particle.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams.

The term “patterning device” used herein should be broadly interpretedas referring to a device that can be used to impart a beam of radiationwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the beam of radiation may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the beam of radiation will correspond to aparticular functional layer in a device being created in the targetportion, such as an integrated circuit.

Patterning devices may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned. In each example of patterning device, thesupport structure may be a frame or table, for example, which may befixed or movable as required and which may ensure that the patterningdevice is at a desired position, for example with respect to theprojection system. Any use of the terms “mask” or “mask” herein may beconsidered synonymous with the more general term “patterning device”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 schematically illustrates redirection of a beam of radiation onan object or on a particle;

FIG. 3 schematically illustrates a beam of radiation leaving an objectbeing detected by a detector system;

FIG. 4 schematically illustrates how a beam of radiation may internallybe refracted, diffracted and/or reflected before leaving the object inthe direction of a detector system;

FIG. 5 schematically illustrates how a beam of radiation may internallyand externally be refracted, diffracted and/or reflected before leavingthe object in the direction of a detector system;

FIGS. 6A-6C schematically illustrate a conventional diaphragm configuredto prevent a first order diffraction pattern being incident on thedetector system;

FIGS. 7A-7C schematically illustrate a diaphragm according to anembodiment of the present invention;

FIG. 8 schematically illustrates a particle detection system includingtwo radiation sources and two radiation detector devices;

FIG. 9 is a circuit diagram of a detection circuit according to anembodiment of the present invention; and

FIGS. 10A and 10C are a schematical side view of an embodiment of aparticle detection system having a radiation blocking assembly;

FIGS. 10B and 10D are a schematical top view of the embodiment of FIGS.10A and 10C, respectively; and

FIG. 10E schematically shows a detector plane of a detector device ofthe embodiment of FIGS. 10A-10D.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL configured to provide a beam PB of radiation(e.g. UV radiation), and a first support structure (e.g. a mask table)MT configured to support a patterning device (e.g. a mask) MA andconnected to a first positioning device PM configured to accuratelyposition the patterning device with respect to the projection system,item PL (“lens”). The apparatus also includes a substrate table (e.g. awafer table) WT configured to hold a substrate (e.g. a resist-coatedwafer) W and connected to a second positioning device PW configured toaccurately position the substrate with respect to the projection system,item PL (“lens”); the projection system (e.g. a refractive projectionlens) PL being configured to image a pattern imparted to the beam ofradiation PB by the patterning device MA onto a target portion C (e.g.including one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD including for example suitabledirecting mirrors and/or a beam expander. In other cases, the source maybe integral part of the apparatus, for example, when the source is amercury lamp. The source SO and the illuminator IL, together with thebeam delivery system BD if required, may be referred to as a radiationsystem.

The illuminator IL may include an adjusting device AM configured toadjust the angular intensity distribution of the beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL generally includes various other components, such as anintegrator IN and a condenser CO. The illuminator provides a conditionedbeam of radiation, referred to as the beam of radiation PB, having adesired uniformity and intensity distribution in its cross-section.

The beam of radiation PB is incident on the mask MA, which is held onthe mask table MT. Having traversed the mask MA, the beam of radiationPB passes through the lens PL, which focuses the beam onto a targetportion C of the substrate W. With the aid of the second positioningdevice PW and position sensor IF (e.g. an interferometric device), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning device PM and another position sensor (which is notexplicitly depicted in FIG. 1) can be used to accurately position themask MA with respect to the path of the beam PB, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe object tables MT and WT will be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the positioning device PM and PW.However, in the case of a stepper (as opposed to a scanner) the masktable MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following modes:

Step mode: the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to the beam ofradiation is projected onto a target portion C at once (i.e. a singlestatic exposure). The substrate table WT is then shifted in the X and/orY direction so that a different target portion C can be exposed. In stepmode, the maximum size of the exposure field limits the size of thetarget portion C imaged in a single static exposure.

Scan mode: the mask table MT and the substrate table WT are scannedsynchronously while a pattern imparted to the beam of radiation isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

Another mode: the mask table MT is kept essentially stationary holding aprogrammable patterning device, and the substrate table WT is moved orscanned while a pattern imparted to the beam of radiation is projectedonto a target portion C. In this mode, generally a pulsed radiationsource is employed and the programmable patterning device is updated asrequired after each movement of the substrate table WT or in betweensuccessive radiation pulses during a scan. This mode of operation can bereadily applied to maskless lithography that utilizes programmablepatterning device, such as a programmable mirror array of a type asreferred to above.

Combinations and/or variations on the above-described modes of use orentirely different modes of use may also be employed.

To show the principle of particle detection by incident radiation andhow artifacts may occur, it is illustrated in FIGS. 2-5 how isotropic,i.e. by a particle or other contamination scattered, and non-isotropic,e.g. diffracted or reflected, radiation may be incident on a detectorsystem.

FIG. 2 shows an object 2 such as a lithographic mask or substrate.Referring to the left-hand side of FIG. 2, a beam 4A hits the surface ofthe object 2. At the location where the beam 4A hits the surface, thenormal 6, i.e. a line perpendicular to the surface, is indicated. Areflection beam 8 may be reflected according to physical laws known tothe person skilled in the art (an exit angle is the same as the angle ofincidence with respect to the normal 6). The incident beam 4A maypartially be refracted, indicated by a refraction beam 10. Depending ona refraction index of the material of object 2 and on the refractionindex of the medium through which the radiation beam 4A travels, therefracted beam 10 is bent towards or away from the normal 6. The amountof radiation being refracted and/or reflected depends on the material ofobject 2, a surface coating of the object 2 and/or on the angle ofincidence, among others.

A detector system 12 detects radiation coming from the location ofincidence of the radiation beam 4A, and being directed towards thedetector system 12, indicated by a detection cone 14. As is seen fromthe left-hand side of FIG. 2, an incident beam 4A is anisotropiclyreflected as a reflection beam 8 and/or anisotropicly refracted as arefraction beam 10. Thus, in this case, no radiation is incident on thedetector and the detector may output a signal having a noise and/or biaslevel, but not having a significant particle detection level.

Now referring to the right-hand side of FIG. 2, a beam of radiation 4Bis incident on a contaminating particle 16 present on the surface ofobject 2. A part of the incident radiation may be absorbed by theparticle 16. Another part may be reflected. Due to the surface shape ofthe surface of the particle 16, the incident radiation is scattered,i.e. isotropicly reflected. Isotropicly reflected radiation, indicatedby arrows 18 is directed in substantially every direction. Therefore, apart of the reflected radiation 18 lies within the detection cone 14 ofthe detector system 12. Thus, the detector system 12 detects radiationand outputs a signal corresponding to the detected radiation having alevel above the particle detection level, i.e. a threshold level.

In FIG. 3, an incident beam of radiation 4 is indicated to hit thesurface of the object 2. From the location of incidence, a beam ofradiation 19 lies within a detection cone 14 of detector system 12 andis incident on the detector system 12. The beam 19 may be radiationhaving been scattered by a contaminating particle, the detection circuitthus correctly detecting the particle.

However, a beam 20 coming from inside the object 2, as a result ofdiffraction, refraction and/or reflection as will be explainedhereinafter, may leave the object 2 and be refracted such that the beam19 results. So, if a beam 20 comes from inside the object 2 having suchan angle with respect to the normal that its refracted beam 19 lieswithin the detection cone 14, the detector system 12 detects radiationwhich was not scattered by a contaminating particle. A detection circuitreceiving a signal from the detector system 12 however determines thatthe signal is above a predetermined threshold level and erroneouslyindicates that a particle is present. Such a detected, but not actuallypresent particle is herein referred to as a ghost particle.

As will be explained in detail below in relation to FIGS. 4 and 5, animportant contributor to the detection of ghost particles is adiffraction pattern. When the object includes a pattern, for example areflective chrome pattern at a surface, at which surface an enteredradiation beam internally diffracts, a diffraction pattern may result.The diffraction pattern may internally reflect and refract and then exitthe object such that at least a part of the diffraction pattern will beincident on the detector. Such a diffraction pattern is an anisotropiccontribution to the radiation incident on the detector.

A diffraction pattern may include a number of orders, i.e. a zero orderpattern, a first order pattern and higher order patterns. A shape,orientation and spacing of the orders of the diffraction pattern isdependent on the shape, orientation and spatial frequency of thediffracting pattern. If a diffracting pattern has a two-dimensionalperiodic structure, the related diffraction pattern will also betwo-dimensional. The energy (intensity) in the diffracted orders dependsamong others on a duty cycle of the pattern (i.e. a spatialcharacteristic of the pattern) and on height differences in the pattern.The direction of the diffraction pattern determines whether none, one ormore orders of the diffraction pattern may reach the detector system.

A part of a diffraction pattern may be blocked by a diaphragm before thepart of the diffraction pattern is incident on the detector. Possibly,another part of the diffraction pattern, such as a second and higherorder diffraction pattern may still pass the diaphragm. According to anembodiment of the present invention, the anisotropic radiation will bediscriminated from the isotropic radiation, since an isotropiccontribution to the radiation will be incident on both parts of thedetector despite of the presence of such a diaphragm. Hence, it may bedetermined whether there is an isotropic contribution to the radiationby verifying that each signal from each part of the detector has asubstantially same (if a center of the detector device and the beam ofradiation are aligned) level higher than a predetermined threshold ornoise level.

The above-described diaphragm splits the detector device in two parts.Despite the presence of the diaphragm, some anisotropic radiation may beincident on the detector device. Moreover, anisotropic radiation may bepartly incident on a first part of the detector system and partlyincident on a second part of the detector system, thereby generating asubstantially same signal level from each detector part. According to afurther embodiment of the present invention a second diaphragm preventssuch incidence by covering an area between parts of the detector system.Thereto, the width of said diaphragm may be equal to or larger than theexpected width of the incident beam of radiation. These embodiments arefurther elucidated below in relation to FIGS. 6A-6D and 7A-7C.

In FIG. 4, it is illustrated how a beam may originate from inside theobject 2. The illustrated object 2 is a mask. On one surface, the mask 2includes a mask pattern 22, which is made, for example, of chromium. Anopposite surface of the mask 2 is scanned for particles. There is noparticle actually present in the case illustrated in FIG. 4.

A beam 4 is directed at and incident on the surface to be scanned. Apart of the incident radiation may be reflected (not shown) and anotherpart may be absorbed and refracted. An absorbed and refracted beam 10travels through the mask 2. At the opposite surface, the refracted beam10 hits the mask pattern 22. The mask pattern 22 is a periodic pattern.Due to the periodicity the radiation is diffracted. A diffracted beam 24travels through the mask 2 and is reflected at the surface of incidence.Subsequent internal reflections at other surfaces may occur, indicatedby reflected beams 26. Eventually, an internally reflected beam 26 mayapproach the location of incidence of the beam 4 such that it leaves themask 2 and is refracted towards the detector system 12 as indicated bythe beam 19. Thus, a ghost particle is detected.

In FIG. 5, again, a refracted beam 19 coming from inside the mask 2 isincident on the detector system 12, but due to another series ofreflections, refractions, and/or diffractions compared to the caseillustrated in FIG. 4. In the case illustrated in FIG. 5, an incidentbeam 4 enters the mask 2 and is refracted as refracted beam 10. Giventhe angle of incidence and the surface conditions, the refracted beam 10is diffracted or refracted at the opposite surface and leaves the mask 2as diffracted beam 28. If the diffracted beam 28 is reflected at asurface 32 of another object, such as a pellicle, a reflected beam 26may hit the surface of the mask 2 again.

The reflected beam 26 is diffracted by periodic mask pattern 22 andenters the mask 2 as a diffracted beam 30. The indicated diffracted beam30 leaves the mask 2 such that it is refracted towards the detectorsystem 12 and is detected. Similar to FIG. 4, a ghost particle isdetected by a detection circuit, although no particle is present.

In the above description in relation to FIGS. 3, 4 and 5, it should benoted that reflection, refraction and diffraction are anisotropic. Theresulting redirected radiation is included in one or more beams asopposed to scattered, isotropic radiation being redirected insubstantially every direction. The multiple diffracted beams are knownas (diffraction) orders.

The cross sections of the beams in a diffraction pattern are dependenton the shape of the cross-section of the incident beam of radiation.When, for instance, the incident beam is a single round beam ofradiation, the diffraction pattern will be a series of single roundbeams.

Further, it is noted that FIGS. 2-5 are for illustrative purpose only,since many other light trajectories are possible. For example, althoughnot shown in FIGS. 2-5, radiation coming from other directions than frominside the illustrated cone 14 may be incident on the detector system12, and thus resulting in the detection of a ghost particle. Otherseries of reflections, refractions and/or diffractions may result inradiation being directed at the detector system such that a detection ofa ghost particle could result.

In FIG. 6A, a surface of a detector system 12 is indicated. The detectorsystem 12 detects radiation incident on the surface and outputs acorresponding signal. A first diffraction pattern 40 including threespots (n^(th), (n+1)^(th) and (n+2)^(th) order) is partially incident onthe detector surface 12. Thus, the detector system 12 detects radiationand outputs a signal corresponding to the intensity of the detectedradiation. Since the diffraction pattern 40 is not radiation scatteredby a particle, a detection circuit receiving the signal erroneouslydetermines the presence of a particle. To prevent detection of a ghostparticle, a diaphragm as shown in FIG. 6B may be used. Such a diaphragmmay be positioned between the surface being scanned and the detector.

Now referring to FIG. 6B, a strip-shaped diaphragm 34 may be used toblock the first diffraction pattern 40. An isotropic reflection,resulting from scattering by a particle is also partially blocked by thestrip-shaped diaphragm 34, however, another part of the isotropicradiation is not blocked and is incident on the detector system 12.Thus, the detector system 12 only detects isotropic scattered radiationand the output signal only results from a present particle.

It is understood that instead of using the strip-shaped diaphragm 34 infront of a detector system 12, a diaphragm 35, as illustrated in FIG.6C, may be employed similarly. A diaphragm opening 37 lets radiationpass. The strip 34 is positioned in the diaphragm opening 37 and blocksthe first diffraction pattern 40.

In FIG. 6D, not only a first diffraction pattern 40 is shown, also asecond diffraction pattern 42 including a number of spots is shown. Thesecond diffraction pattern 42 may result from another part of the maskpattern 22 (see FIGS. 4 and 5) or may result from other characteristicsof the diffracting pattern. The strip-shaped diaphragm 34 blocks thefirst diffraction pattern 40, but the second diffraction pattern 42passes the diaphragm and reaches the detector system 12. Thus, it isstill possible that a ghost particle is detected, since a seconddiffraction pattern 42 may lead to a signal indicating the presence of aparticle.

In FIGS. 7A-7C, a cross-shaped diaphragm comprising a vertical diaphragm34 and a horizontal diaphragm 36 is shown. Like in FIG. 6D, the verticaldiaphragm 34 blocks the (not shown) first diffraction pattern. Thesecond diffraction pattern 42 may have a number of positions. The widthof the diaphragms 34, 36 is selected such that the width is larger thanthe width of the spots of the diffraction pattern 42.

In FIG. 7A, the second diffraction pattern 42 is blocked by thehorizontal diaphragm 36. Thus, no diffracted beam is incident on thedetector system 12. In FIG. 7B however, the second diffraction pattern42 is not blocked, but passes the diaphragm 34, 36, at least partially,and diffracted radiation is incident on the detector system 12. The sameapplies to the situation shown in FIG. 7C, in which situation thediffraction pattern 42 passes on an opposite side of the horizontaldiaphragm 36 compared to the situation illustrated in FIG. 7B.

When the situation shown in FIG. 7A occurs, no ghost particle isdetected, since no diffracted radiation is incident on the detector 12.Only isotropic, scattered radiation may be detected. In FIGS. 7B and 7C,the anisotropic diffracted radiation 42 is incident on only one half ofthe detector system 12: the lower half or the upper half, respectively.Consequently, the other half of the detector system 12 only detectsscattered radiation, which is also detected by the other half of thedetector system 12 apart from the diffracted radiation. Thus, if twosignals corresponding to radiation detected on a corresponding half ofthe detector system 12, are output by the detector system 12 anddiffracted radiation is present, one signal has a higher level than theother due to the contribution of the anisotropic diffracted radiation.

Three situations may occur:

1. Both non-isotropic (diffracted, refracted, reflected) radiation andisotropic (scattered) radiation is incident on the detector system 12;

2. Only isotropic (scattered) radiation is incident on the detector; or

3. Only non-isotropic (diffracted, refracted, and/or reflected) isincident on the detector.

In the first situation, both signals have a level higher than apredetermined noise (threshold) level, indicating that at leastisotropic radiation was incident on the detector system 12, and adetection circuit may determine that a particle is present. In thesecond situation, both halves of the detector output a substantiallysame signal level, which indicates the presence of only a particle,without a ghost particle contribution. In the third situation, only oneof the signals has a level above the noise (threshold) level, indicatingthat the detected radiation is due to diffraction, refraction and/orreflection, but not to scattering by a particle. A detection circuit maydetermine that no particle is present.

Another embodiment of the present invention does not depend on spatialdiscrimination but depends on wavelength related discrimination. Asmentioned above, an important contributor to the detection of ghostparticles is a diffraction pattern. However, the diffraction patternsare dependent on the wavelength of the incident radiation beingdiffracted. Using two, or even more radiation sources having each theirown specific wavelength, different diffraction patterns are generated.Using a detector for each wavelength, the diffraction pattern for eachwavelength is detected by the respective detectors. The choice ofwavelength is made such that it is highly improbable that differentdiffraction patterns, and also other internal reflections andrefractions, of the radiation beams of different wavelengths will allreach the corresponding detectors. Thus, similar to the detection methodof the above described embodiment, it may be determined whether there isan isotropic contribution to the detected radiation by verifying thateach signal from each detector has a level higher than a noise level.

It is noted that the first and second detector may physically berepresented by one detector of which the detected signal may be splitinto a number of contributions coming from the different beams ofradiation having different wavelengths. The number of contributions maybe determined after simultaneous detection of the number of beams, orthe number of contributions may be determined one after another, forexample using pulsed radiation sources.

FIG. 8 illustrates the above-mentioned embodiment of the presentinvention, wherein two radiation sources 38A and 38B output two beams ofradiation 4A and 4B having different wavelengths. The wavelengths areselected such that it is unlikely that possibly resulting diffractionpatterns overlap in their trajectory towards the correspondingdetectors. An optical device 44A combines the two radiation beams 4A and4B and the combined beam is directed at the surface of the object to bescanned. The combined beam may pass through other optical devices 46Aand may hit a particle 16, or not.

After being scattered, reflected, refracted and/or diffracted, radiationmay travel through a number of optical devices 46B, if present. Then, anoptical device 44B splits the radiation into radiation havingwavelengths in the range corresponding to beam 4A and radiation havingwavelengths in the range corresponding to beam 4B to route the radiationto its corresponding detector 12A or 12B.

The beams 4A and 4B have different wavelengths as mentioned above. Eachbeam 4 may be a beam of monochrome radiation or including radiationhaving wavelengths within a certain range of wavelengths. Thewavelengths may be different to ascertain that the beams will havedifferent diffraction patterns, since the diffraction patterns aredependent on the wavelength of the incident radiation. When incidentradiation beams 4A and 4B enter the object (thus they do not hit aparticle), they may be diffracted, reflected and/or refracted such thata part of the incident radiation leaves the object in the direction of adetector 12A or 12B, respectively. Since their diffraction patterns aredifferent due to their different wavelengths, it is virtually impossiblethat both detectors 12A and 12B receive such anisotropic radiationsimultaneously. Thus, if radiation from the beams 4A and/or 4B isdiffracted, reflected and/or refracted, only one of the respectivedetectors 12A and 12B may output a signal having a level above apredetermined threshold level, and therefore it is determined that noparticle is present.

When the beams 4A and 4B hit a particle 16, both beams 4A and 4B arebeing scattered and from both beams 4A and 4B a part of the radiation isscattered/reflected in the direction of their respective detectors 12Aand 12B. Thus, when a particle is present, both detectors 12A and 12Breceive an amount of radiation and output a signal corresponding to thereceived amount of radiation having a level above a predetermined noiselevel, and therefore it is determined that a particle is present. It isnoted that, in this embodiment, the first and second detector mayphysically be represented by one detector of which the detected signalmay be split into a number of contributions coming from the differentbeams of radiation having different wavelengths as mentioned above.

If a particle is present and anisotropicly redirects radiation, and ifisotropicly redirected radiation is also incident on one of thedetectors, both detectors output a signal having a level above thethreshold level. However, one signal has a level representinganisotropicly and isotropicly redirected radiation and another signalhas a level representing only anisotropicly redirected radiation.Despite the difference in signal level, the contaminating particle isdetected, since both signals have a level above the threshold level, andtherefore it is determined that a particle is present. Due to the choiceof wavelengths, it is unlikely that both wavelengths will result in anisotropic (ghost) signal in their corresponding detector, and thusreliably discriminating a particle signal from a ghost signal.

In each of the above-mentioned embodiments of the present invention, twoor more independent signals are verified to have a level higher than athreshold level. If all levels are higher than the threshold level, thedetector circuit determines that a particle is present on the surface,since it is thus detected that isotropic light is incident on thedetector. Therefore, in an embodiment of the present invention asillustrated in FIG. 9, the detection circuit includes a number ofcomparators configured to compare a corresponding number of inputsignals with a threshold level, each comparator outputting a logicalcomparator signal; an AND-operator configured to receive each logicalcomparator signal and to output a logical TRUE signal when each logicalcomparator signal represents TRUE. Thus, when a particle is present onthe surface, the detection circuit outputs a logical TRUE.

FIG. 9 illustrates a detection circuit for use with the above-describedembodiments of the present invention to determine whether a particle ispresent on a surface, or not. By verifying that a number of signals (inthe shown embodiment of the detection circuit two signals) have a levelabove a threshold level representing a predetermined noise level, it isdetermined whether a particle is present. The exemplary circuitillustrated in FIG. 9 is a digital circuit. The circuit may however aswell be an analogue circuit.

Referring to FIG. 9, a first detector system 12A outputs a signal to anamplifier 50A. The output of the amplifier 50A is input in ananalog-to-digital converter 52A. The amplifier 50A is however not anessential component of the detection circuit, since the output of thedetector system 12A may be directly input in the analog-to-digitalconverter 52A. An output of the analog-to-digital converter 52A is fedto a comparator 54A as a first input and a threshold signal 56Arepresenting a predetermined noise level is input in the comparator 54Aas a second input. The comparator 54A compares the input signals andoutputs a logical comparison signal 58A. The logical comparison signal58A is input as a first input in an AND-operator 60.

The above described branch of the detection circuit may be present anynumber of times. In the embodiment shown in FIG. 9, there is a secondbranch including detector system 12B, amplifier 50B, analog-to-digitalconverter 52B, comparator 54B, threshold signal 56B and logicalcomparison signal 58B, which signal 58B is also input in AND-operator60. Similarly, any number of identical branches may be present in thedetection circuit. The AND-operator 60 then has a corresponding numberof inputs. A logical AND-operator output 62 represents a logical valueindicating whether a particle is present, or not.

The detected amount of scattered radiation is a measure of a size of adetected particle. Therefore, the detection circuit may be adapted notonly to indicate whether a particle is present, but also to indicate anestimate of the size of the particle. The logical AND-operator output 62however only indicates whether a particle is present, since the logicalvalue can only represent ‘TRUE’ or ‘FALSE’. To indicate a size, thesignal value of one or both of the detectors should be preserved andpresented at an output of the detection circuit, if a particle isdetected. The detection circuit shown in FIG. 9 is configured to outputthe size if a particle is detected, and to output a NULL-signal, if noparticle is detected.

Thereto, the detection circuit is provided with an ADD-operator 66,which adds the output of each analog-to-digital converter, in theillustrated embodiment converters 52A and 52B. If more branches arepresent in the circuit, the ADD-operator 66 has a corresponding numberof inputs and adds all input signals. From an output of the ADD-operator66, an ADD-operator output signal 68 is fed to an MUL-operator 70multiplying input signals. Beside the ADD-operator output signal 68, thelogical AND-operator signal 62 is input in the MUL-operator 70. TheMUL-operator output 72 represents the size of a particle if detected,and NULL if no particle is detected, as will be explained below.

A person skilled in the art will from the below description readilyunderstand how the detection circuit functions. It will be appreciatedthat the circuit may be extended to any number of detector signals.Detectors 12A and 12B output a signal representing the amount ofradiation received by the detector system 12A and 12B, respectively. Theoutputs may be amplified by amplifiers 50A and 50B, respectively, andmay be digitized by analog-to-digital converters 52A and 52B,respectively. Alternatively, digital signals S1 and S2 may be processedusing an algorithm implemented in software. In software, more complexalgorithms may easily be programmed and used, for example. The detectioncircuit may be analog as well and therefore the converters 52A and 52Bmay be omitted.

The output of the converters 52A and 52B are thus still representing thesignal value of the output of the detectors 12A and 12B, respectively.The output of the converters 52A and 52B are input in the respectivecomparators 54A and 54B that compare the signal value with a respectivethreshold value 56A and 56B to determine whether the detector outputshave a level above the threshold. The threshold value 56A, 56Brepresents a predetermined noise level. If the detector output liesunder the noise level, the detector system 12A, 12B did not receive acertain minimum amount of radiation and the output may be assumed to beno more than noise. The comparator 54A, 54B outputs a logical signal58A, 58B representing ‘TRUE’ (‘1’) if the detector output is higher thanthe threshold value 56A, 56B, and representing ‘FALSE’ (‘0’) if thedetector output is lower than the threshold value 56A, 56B.

If both comparators 54A and 54B output a logical TRUE (‘1’), theAND-operator 60 receives only signals 58A, 58B representing TRUE andtherefore outputs a logical TRUE (‘1’). If one or none of the inputs58A, 58B represents FALSE (‘0’), the output 62 of the AND-operator 60represents FALSE (‘0’).

The ADD-operator 66 adds the detector outputs independent from whether aparticle is detected, or not. Adding the detector outputs isparticularly suited for the embodiment described in relation to theembodiment of FIG. 7, since in that embodiment the received amount ofradiation is divided over two halves of the detector and is output astwo separate signals. The two separate signals added together thenrepresents the total received amount of radiation. The two detectors 12Aand 12B shown in FIG. 9 should in that case be interpreted as two halvesof one detector. Thus, the ADD-operator output signal 68 represents atotal received amount of radiation.

In an embodiment of the invention, the detection circuit may beconfigured to determine a size of a detected particle based on thedetector signal. The level of the detector signal represents anintensity of the detected radiation incident on the detector. Theintensity of the incident radiation is a measure for the size of thedetected particle.

The ADD-operator output signal 68 is input in the MUL-operator 70. TheMUL-operator 70 further receives a logical TRUE (‘1’) or FALSE (‘0’)when a particle is present and when a particle is not present,respectively. Thus, when a particle is present, the MUL-operator 70multiplies the ADD-operator output signal 68 and the logical TRUE (‘1’)resulting in an output signal 72 having the same signal value as theADD-operator output signal 68. The ADD-operator output signal 68represents the amount of radiation received by the detectors 12A and12B, and thus is a measure of the size of a detected particle. Theoutput 72 of the MUL-operator 70 therefore represents the size of adetected particle, since, if no particle is detected, the ADD-operatoroutput signal 68 is multiplied by 0 (‘FALSE’) and would represent NULL.

When receiving more than one signal, one from each part of a detector asdescribed above in relation to one or more embodiments of the presentinvention, or one or more signals from more than one detector, eachseparate signal or a combination of two or more signals may be used todetermine the particle size. A suitable combination may be, for example,the sum of two signals coming from one detector, thus reconstructing theintensity of radiation incident on the whole detector.

Light incident on a detector may include both isotropically andanisotropically redirected light, i.e. light redirected by a particleand light representing a ghost particle. Thus, both signals may have alevel higher than the threshold level, while a first signal may have asubstantially higher level than a second signal. In such a case, thefirst signal is generated due to both the isotropic contribution and theanisotropic contribution to the incident light. The second signal onlyrepresents the isotropic contribution, i.e. the radiation resulting fromscattering by a particle. Therefore, in an embodiment of the detectioncircuit, the signal having the lowest level may be used for determiningthe particle size.

Instead of using the ADD-operator 66, it will be appreciated that anyother circuitry may be employed to output a signal representing a sizeof a particle. For example, it may be considered that if both detectors12A and 12B output a high signal, one of them may have received not onlyscattered isotropic radiation, but also non-isotropic radiation.Therefore, one signal may be substantially higher than the other. Insuch a case, it is considered to use only the lower signal, since it isassumed that the lower signal represents only scattered radiation andmay thus be more representative for the size of the scattering particle.A person skilled in the art readily understands how a circuit may bebuilt using logical operators or analogue circuitry, which circuitselects the lower signal and outputs the signal to the MUL-operator 70shown in FIG. 9.

In order to detect reticle contamination and to measure size ofcontaminant particles, internal or external reticle inspection systemsmay be used. These systems may contain a digital camera or a scanner toobtain a two-dimensional image of (or part of) the surface of thereticle.

Estimating the size of particles which are smaller than the resolution(pixel size or spot size of the scanning beam) of the two-dimensionalimaging system can be done by measuring the maximum reflected signalintensity (pixel intensity) when the particle is exposed with light andby comparing it with the amount of light reflected by standard latexspheres of known dimensions.

It is also possible to extract particle features from thetwo-dimensional bitmap, like shape and size. These two-dimensionalbitmaps can be used to estimate the size of particles that are largerthan the resolution of the two-dimensional imaging system. This methodcan be referred to as Image feature method. This method is generallyaccurate only for particles that are significantly larger than theresolution of the two-dimensional imaging system. In a practicalembodiment, the latter may be about 70 μm.

Although the latex equivalent sphere (LES) reflection method works wellfor contaminant particles that are smaller than the resolution of thetwo-dimensional imaging system, this method may not be desirable forlarger particles because it may result in unrealistic estimates,basically classifying those particles as being too small. The range oflatex spheres used in the reflection method is generally limited to from10 to 100 μm.

Since particles are detected on the top surface of the reticle and/or onthe bottom surface of the pellicle (hard or soft), particles may alwaysbe at an out-of-focus position with respect to the pattern on thereticle, which implies that small particles are not as relevant as largeparticles since they have less impact on imaging. Therefore, thecontamination detection method of choice should also correctly estimatethe size of particles larger than about 60 μm.

In an embodiment of the invention, a new method for detecting particleson an object is presented. First, a two-dimensional image of the reticleis obtained. This two-dimensional image contains particle reflectionintensities. Then, individual particles are located by scanning thisimage for clusters of connected pixels with an intensity above a certainthreshold. The size of each pixel cluster may be determined according tothe image feature method but if this size is less than a predefinedvalue, then the latex equivalent sphere reflection method may be usedinstead. The particle size according to the image feature method isextracted from the image by determining, in an embodiment of theinvention, the width and height of a rectangular bounding box includingall cluster pixels. Alternatively, the number of cluster pixels, thediagonal of the bounding box, or the maximum distance between 2 clusterpixels can be used, for example. It will be appreciated that a particlemay be strongly asymmetric (e.g. the width of the bounding box is muchlarger than its height) in which case the image feature method may beused.

It will be appreciated that the resulting method may be capable ofdetecting both small and large particles, where the range is in practiceonly limited by the size of the two-dimensional image.

Another embodiment of the present invention depends on spatialdiscrimination on the surface of the detector device. Referring to FIGS.10A and 10B, illustrating this embodiment of the present invention, aradiation blocking assembly 130 is positioned in a radiation trajectory120 (optical axis) from the surface 102A of an object 102 to a radiationdetector device 112. The radiation blocking assembly 130 comprises alens 131 and a diaphragm 132. A contaminating particle 116 is present onthe surface 102A of the object 102. A beam of radiation 110 is incidenton the particle 116. A resulting scattered beam of radiation redirectedby the particle 116 is shown as cone 114 propagating towards thedetector device 112.

The lens 131 is configured to focus radiation coming from a radiationsource in a plane of the surface 102A of the object 102 in a plane ofthe detector device 112. Thus, the beam of radiation 114 scattered bythe particle 116 is focused onto the detector device 112. The diaphragm132, however, blocks a part of the radiation incident on the lens 131. Apart of the radiation of cone 114 passes the diaphragm 132 and is stillbeing focused on detector device 112. An effect resulting from thediaphragm 132 for the image of the contaminating particle 116 is a lossof radiation intensity on the detector device 112.

Now referring to FIGS. 10C and 10D, further illustrating theabove-mentioned embodiment, the diaphragm 132 is positioned in aradiation trajectory 120 (optical axis) from the surface 102A of anobject 102 to the radiation detector device 112.

In FIGS. 10C and 10D, no contaminating particle is present on thesurface 102A of the object 102. A beam of radiation (not shown) isincident on the surface 102A. The beam has been refracted into theobject 102 and is in the object 102 reflected and possibly diffracted ata pattern surface 102B. A resulting beam of diffracted radiation insidecone 118 is shown as beam 119. The cone 118 may be regarded as anassembly of possible anisotropic beams of radiation coming from a ghostparticle 122 and directed toward lens 131.

The diaphragm 132 blocks a part of the isotropic radiation directedtoward the lens 131. The part of the radiation of cone 114 passing thediaphragm 132 is being focused on the detector device 112. The effectresulting from the diaphragm 132 for the image of the particle 116A is aloss of radiation intensity on the detector device 112.

When an anisotropic beam is generated in the pattern surface 102B thenthe point where this beam leaves surface 102B can be regarded as thelocation of a ghost particle 122. Due to refraction at surface 102A theghost beam will have a different direction when it leaves object 102,and it will seam that the ghost beam has its origin in the virtual ghostparticle 122A as drawn in FIG. 10C. If the beam reaches lens 131 it willbe located inside cone 118. Regarding an anisotropic beam coming fromthe ghost particle 122 inside cone 118 there are two possibilities:either the beam is blocked by diaphragm 132 or the beam (partly) passesdiaphragm 132, is incident on the lens 131 and is focused in point 117.Point 117 is the image of ghost particle 122 made by lens 131. Becauseghost particle 122A is located further away from lens 131 than particle116, the image in point 117 is located at some distance in front of theimage of particle 116 on the detector device 112 according to the lawsof optical imaging. After passing the point 117 the radiation from ghostparticle 122 propagates further towards the detector device 112 andresults in a spot in area 112B or 112C of detector device 112, outsidearea 112A.

In FIG. 10E the areas 112A, 112B and 112C are depicted on a largerdrawing scale. Area 112A can be described as a shadow of diaphragm 132made by all possible beams in cone 118 that come from particle 122,focused in point 117. Hence, no radiation resulting from a ghostparticle 122 is present in a part 112A of the detector device 112. Thepart 112A of the detector device 112 may be referred to hereinafter as ano-ghost detection part 112A of the detector device 112. Therefore inthe situation drawn in FIG. 10A to 10D signals from area 112A are onlyrelated to particle 116 and not to ghost particle 122.

However, in exceptional cases, through double diffraction ontwo-dimensional patterns, a ghost particle may appear to lie out of theplane of drawing 10C; in such a case a ghost particle may enter area112A, and 112B or 112C. To prevent an incorrect determination of thepresence of a particle, in an embodiment, signals from area 112B or 112Crelated to ghost particle 122 may be employed. An image 133 of particle116 on detector device 112 (FIG. 10E) is within area 112A. Therefore,signals from area 112B or 112C are only related to a ghost particle 122and not to an actual particle 116. Signals from area 112B or 112C can beused to warn for the presence of a ghost particle 122. This is usefulfor the above-described situation of a ghost particle located out of theplane of drawing 10C where a signal from area 112A could bemisinterpreted as a signal from a particle 116 while the signal actuallyis related to a ghost particle. Thus, the signals from area 112B or 112Cenable to correct a misinterpretation.

The size of the no-ghost detection part 112A is dependent on thedistance of the ghost particle 122 and a focal point of the lens 131.The focal point of the lens 131 is selected such that the radiationscattered by a contaminating particle 116 is focused on a part 112A ofthe detector device 112. The size of the no-ghost detection part 112Adepends, inter alia, on the focal point of the lens 131, a distancebetween the detector device 112 and the point 117 and a size of thediaphragm 132.

It is noted that a person skilled in the art may combine the embodimentas illustrated in FIGS. 10A to 10E, and described above, with theembodiment as described in relation to FIGS. 6-9 without using anyinventive skills. For example, the diaphragm 132 may have a shape asshown in FIG. 6A-6D or FIG. 7A-7C. In such an embodiment, a width of thediaphragm 132 may be, in side view and/or in top view, substantiallyequal to a width of the lens 131.

While the invention has been described and illustrated in its preferredembodiments, it should be understood that departures may be madetherefrom within the scope of the invention, which is not limited to thedetails disclosed herein.

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which can be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure. Further, the terms and phrases usedherein are not intended to be limiting; but rather, to provide anunderstandable description of the invention.

The terms “a” or “an,” as used herein, are defined as one or more thanone. The term “plurality,” as used herein, is defined as two or morethan two. The term another, as used herein, is defined as at least asecond or more. The terms “including” and/or “having,” as used herein,are defined as comprising (i.e., open language). The term “coupled,” asused herein, is defined as connected, although not necessarily directly,and not necessarily mechanically.

1. A particle detection system configured to detect a particle on asurface of an object, the system comprising: a first radiation sourceconfigured to generate a beam of radiation having a first wavelength,the beam of radiation being directed to a detection area at the surfaceof the object; a radiation detector system configured to output aplurality of detector signals corresponding to an intensity of radiationreceived from the detection area incident on the radiation detectorsystem; and a detection circuit coupled to the radiation detector systemand configured to determine from the plurality of detector signalswhether a particle is present on the surface of the object, whereinpresence of a particle is determined by resolving a signal from a ghostparticle relative to a signal from the particle.
 2. The particledetection system of claim 1, wherein the detection circuit is configuredto compare each of the plurality of detector signals with apredetermined threshold level, and to indicate that a particle ispresent on the surface of the object, when each of said plurality ofdetector signals has a level above said threshold level.
 3. The particledetection system of claim 2, further comprising a cross-shaped diaphragmpositioned between the surface of the object and the radiation detectorsystem, wherein the radiation detector system comprises a detectordevice, said detector system being configured to output a first detectorsignal corresponding to the intensity of radiation incident on a firstpart of the detector system, and to output a second detector signalcorresponding to the intensity of radiation incident on a second part ofthe detector system.
 4. The particle detection system of claim 2,further comprising a second radiation source configured to generate abeam of radiation having a second wavelength, the beam of radiationbeing directed to the surface of the object, wherein the detector systemcomprises a first detector device configured to output a first detectorsignal corresponding to an intensity of radiation having the firstwavelength and being incident on the first detector, and a seconddetector device configured to output a second detector signalcorresponding to an intensity of radiation having the second wavelengthand being incident on the second detector device.
 5. The particledetection system of claim 2, wherein the detector system comprises (1) afirst detector subsystem, the first detector subsystem being configuredto output a first detector signal corresponding to the intensity ofradiation incident on a first part of the first detector subsystem, andto output a second detector signal corresponding to the intensity ofradiation incident on a second part of the first detector subsystem, and(2) a second detector subsystem, the second detector subsystem beingconfigured to output a third detector signal corresponding to theintensity of radiation incident on a first part of the second detectorsubsystem, and to output a fourth detector signal corresponding to theintensity of radiation incident on a second part of the second detectorsubsystem; and wherein the system further comprises a cross-shapeddiaphragm positioned between the surface of the object and eachdetector.
 6. The particle detection system of claim 2, wherein thedetection circuit comprises a plurality of comparators configured tocompare a corresponding plurality of input detector signals with athreshold level, each of the plurality of comparators outputting alogical comparator signal, and an AND-operator configured to receiveeach logical comparator signal and to output a logical TRUE signal wheneach of the plurality of input signals is greater than the thresholdlevel.
 7. The particle detection system of claim 2, wherein thedetection circuit is further configured to determine a size of adetected particle based on the detector signal.
 8. The particledetection system of claim 7, wherein the size of the detected particleis determined based on one detector signal or a combination of at leasttwo detector signals.
 9. The particle detection system of claim 7,wherein the radiation detector system comprises a detector devicecomprising an array of detector pixel elements, each detector signal ofthe plurality of detector signals corresponding to an intensity ofradiation incident on at least one detector pixel element, therebyproviding a particle image of a detected particle, the image comprisinga number of image pixels, wherein the detection circuit is configured todetect whether a detected particle is small or large based on theparticle image; determine the size of a small particle based on theradiation intensity incident on at least one detector pixel element; anddetermine the size of a large particle based on image features of theparticle image.
 10. The particle detection system of claim 9, whereinthe detection circuit is configured to detect that a particle is smallif a total number of image pixels relating to the detected particle issmaller than a predetermined number of pixels.
 11. The particledetection system of claim 4, wherein a size of the particle isdetermined based on the signal from the first detector, the signal fromthe second detector or a combination of the signals from the first andthe second detector.
 12. The particle detection system of claim 1,wherein the radiation detector system comprises a radiation detectordevice for generating the first detector signal in response to radiationincident on at least one predetermined part of the radiation detectordevice; and a radiation blocking assembly for preventing radiation notoriginating from the detection area around the surface of the objectfrom being incident on the predetermined part of the radiation detectordevice.
 13. The particle detection system of claim 12, wherein theradiation blocking assembly comprises a detector lens for focussingradiation originating from the detection area on the at least onepredetermined part of the radiation detector device; a blocking devicefor preventing radiation not originating from the detection area frombeing directed by the detector lens onto the predetermined part of thedetector device.
 14. The particle detection system of claim 12, whereinthe radiation detector system is configured to further output at least asecond detector signal for indicating whether radiation is incident onthe radiation detector device outside said at least one predeterminedpart of the radiation detector device for detecting a ghost particlesignal.
 15. The particle detection system of claim 12, wherein thedetection circuit is further configured to determine a size of adetected particle based on the detector signal.
 16. The particledetection system of claim 15, wherein the detector system comprises adetector device comprising an array of detector pixel elements, eachdetector signal of the plurality of detector signals corresponding to anintensity of radiation incident on at least one of said detector pixelelements, the plurality of detector signals thereby providing a particleimage of a detected particle, the image comprising a number of imagepixels, wherein the detection circuit is configured to detect whether adetected particle is small or large based on the particle image;determine the size of a small particle based on the radiation intensityincident on at least one detector pixel element; and determine the sizeof a large particle based on image features of the particle image. 17.The particle detection system of claim 16, wherein the detection circuitis configured to detect that a particle is small if a total number ofimage pixels relating to the detected particle is smaller than apredetermined number of pixels.
 18. A lithographic apparatus comprising:an illumination system configured to condition a beam of radiation; asupport structure configured to support a patterning device, thepatterning device serving to impart the beam of radiation with a patternin its cross-section; a substrate table configured to hold a substrate;a projection system configured to project the patterned beam onto atarget portion of the substrate, and a particle detection systemconfigured to detect a particle in a detection area at a surface of anobject, wherein presence of a particle is determined by spatiallyresolving a signal from a ghost particle from a signal from theparticle.
 19. The lithographic apparatus of claim 18, wherein saidparticle detection system comprises a detection circuit configured tocompare a plurality of detector signals with a predetermined thresholdlevel, and to indicate that a particle is present on the surface of theobject, when said plurality of detector signals has a level above saidthreshold level.
 20. The lithographic apparatus of claim 19, theparticle detection system further comprising a cross-shaped diaphragmpositioned between the surface of the object and the detector system,wherein the radiation detector system comprises a detector, saiddetector being configured to output a first detector signalcorresponding to the intensity of radiation incident on a first part ofthe detector, and to output a second detector signal corresponding tothe intensity of radiation incident on a second part of the detector.21. The lithographic apparatus of claim 19, the particle detectionsystem further comprising a second radiation source configured togenerate a beam of radiation having a second wavelength, the beam ofradiation being directed to the surface of the object, wherein thedetector system comprises a first detector configured to output a firstdetector signal corresponding to an intensity of radiation having thefirst wavelength and being incident on the first detector, and a seconddetector configured to output a second detector signal corresponding toan intensity of radiation having the second wavelength and beingincident on the second detector.
 22. The lithographic apparatus of claim19, wherein the detector system of the particle detection systemcomprises a first detector, the first detector being configured tooutput a first detector signal corresponding to the intensity ofradiation incident on a first part of the first detector, and to outputa second detector signal corresponding to the intensity of radiationincident on a second part of the first detector, and a second detector,the second detector being configured to output a third detector signalcorresponding to the intensity of radiation incident on a first part ofthe second detector, and to output a fourth detector signalcorresponding to the intensity of radiation incident on a second part ofthe second detector; and wherein the system further comprises across-shaped diaphragm positioned between the surface of the object andeach detector.
 23. The lithographic apparatus of claim 19, wherein thedetection circuit of the particle detection system comprises a pluralityof comparators configured to compare a corresponding plurality of inputdetector signals with a threshold level, each of the plurality ofcomparators outputting a logical comparator signal, and an AND-operatorconfigured to receive each logical comparator signal and to output alogical TRUE signal when each of the plurality of input signals isgreater than the threshold level.
 24. The lithographic apparatus ofclaim 18, wherein the detection circuit is further configured todetermine a size of a detected particle based on the detector signal.25. The lithographic apparatus of claim 24, wherein the size of thedetected particle is determined based on one detector signal or acombination of at least two detector signals.
 26. The lithographicapparatus of claim 24, wherein the detector system comprises a detectordevice comprising an array of detector pixel elements, each detectorsignal of the plurality of detector signals corresponding to anintensity of radiation incident on at least one detector pixel element,thereby providing a particle image of a detected particle, the imagecomprising a number of image pixels, wherein the detection circuit isconfigured to: detect whether a detected particle is small or largebased on the particle image; determine the size of a small particlebased on the radiation intensity incident on at least one detector pixelelement; and determine the size of a large particle based on imagefeatures of the particle image.
 27. The lithographic apparatus of claim26, wherein the detection circuit is configured to detect that aparticle is small if a total number of image pixels relating to thedetected particle is smaller than a predetermined number of pixels. 28.The lithographic apparatus of claim 21, wherein a size of the particleis determined based on the signal from the first detector, the signalfrom the second detector or a combination of the signals from the firstand the second detector.
 29. The lithographic apparatus of claim 18,wherein said object is the substrate or the patterning device.
 30. Thelithographic apparatus of claim 18, the particle detection systemcomprising: a radiation detector device for generating a first detectorsignal in response to radiation incident on at least one predeterminedpart of the radiation detector device; and a radiation blocking assemblyfor preventing radiation not originating from the detection area aroundthe surface of the object from being incident on the predetermined partof the radiation detector device.
 31. The particle detection system ofclaim 30, wherein the radiation blocking assembly comprises a detectorlens for focussing radiation originating from the detection area on theat least one predetermined part of the radiation detector device; ablocking device for preventing radiation not originating from thedetection area from being directed by the detector lens onto thepredetermined part of the detector device.
 32. The particle detectionsystem of claim 31, wherein the radiation detector system is configuredto further output at least a second detector signal for indicatingwhether radiation is incident on the detector device outside said atleast one predetermined part of the detector device for detecting aghost particle signal.
 33. A device manufacturing method comprising:projecting a patterned beam of radiation onto a target portion of asubstrate, and detecting a particle on a surface of an object with aparticle detection system, wherein said particle detection systemcomprises a detection circuit, which is configured to compare aplurality of detector signals with a predetermined threshold level, andto indicate that the particle is present on the surface of the object,when said plurality of detector signals has a level above said thresholdlevel.
 34. The device manufacturing method of claim 33, wherein saidobject is the substrate or a patterning device used to pattern the beamof radiation.
 35. A device manufacturing method comprising: projecting apatterned beam of radiation onto a target portion of a substrate, anddetecting a particle on a surface of an object with a particle detectionsystem, wherein the particle detection system comprises a radiationdetector device for generating the first detector signal in response toradiation incident on at least one predetermined part of the radiationdetector device; and a radiation blocking assembly for preventingradiation not originating from a detection range around the surface ofthe object from being incident on the predetermined part of the detectordevice.
 36. The device manufacturing method of claim 35 wherein saidobject is the substrate or a patterning device used to pattern the beamof radiation.
 37. A device manufacturing method comprising: projecting apatterned beam of radiation onto a target portion of a substrate, anddetecting a particle on a surface of an object, said detectingincluding: providing a beam of radiation onto the surface of saidobject, detecting the beam of radiation redirected by said particleand/or said object; outputting a plurality of signals corresponding toan intensity of the detected beam of radiation; and comparing theplurality of signals with a predetermined threshold level to determinewhether a particle is present on the surface of said object.
 38. Adevice manufacturing method comprising: projecting a patterned beam ofradiation onto a target portion of a substrate, and detecting a particleon a surface of an object, said detecting including: providing a beam ofradiation onto the surface of said object, detecting the beam ofradiation redirected by said particle and/or said object; directing abeam of radiation originating from within a detection range around thesurface of the object on a predetermined part of a radiation detectiondevice; preventing radiation not originating from within a detectionrange around the surface of the object from being incident on thepredetermined part of a radiation detection device; and determiningwhether a particle is present on the surface of said object based on adetector signal corresponding to an intensity of the detected beam ofradiation incident on the predetermined part of the radiation detectiondevice.